The proposed research is a study of the extent and control of metabolism by the mechanical output during portions of a single contraction of skeletal muscle. The extent of energy yielding reactions seems to agree quite well with the heat and work in series of complete contraction cycles. Serious discrepancies were recently noted in the case of a single contraction: the extent of known metabolic reactions is insufficient to account for the amount of physical energy put out; a goal is to sort out the basis for these differences. Very little ATP is used in an unloaded contraction although the heat is considerable; rather the metabolic flux is closely governed by the work output. This requires the existence of at least one other process or reaction, spontaneously reversing during relaxation, to achieve energy balance over the full cycle. Although a search for the source(s) of the heat may be possible, certain assumptions in their interpretation may well be invalid. The strategy of this proposal involves direct metabolic and mechanical measurements and is based on the view that the mechanical output controls the energy metabolism. The regulation and extent of ATP utilization will be directly studied at different constant velocities of shortening and over a range of muscle lengths during the contraction cycle by exploiting the infrequently used isovelocity contraction. Other experiments will measure the magnitude and course of other (quantitatively less significant) metabolic energy sinks, e.g. the chemical work of ion movements. These experiments have the advantage of being interpreted independently of heat analyses, greater specificity and being directly related to the contractile process itself.